In electrical equipment having multiple modules or units that are interconnected, it is desirable to provide a keying system that uniquely defines plug and receptacle pairs that are to be mated. There are many such systems in use in the industry. Such systems, however, are typically molded into the walls of the mating connector halves by adding to the wall thickness or by simply adding the key structure to the surface of an existing wall. An example of the first approach is illustrated in U.S. Pat. No. 4,764,129 which discloses mating electrical connector housings having plug keys that vary in width and corresponding receptacle openings that also vary in width. An example of the second approach is illustrated in U.S. Pat. No. 4,376,565 which discloses mating electrical plug and receptacle housings having keying structures added to the outside of the plug wall and a corresponding projection on the receptacle wall to accommodate the necessary opening for receiving the plug key. Other keying arrangements utilize separate key structures that are added to the connector halves to effect the desired unique keying. An example of this keying approach is illustrated in U.S. Pat. No. 5,044,994 which discloses a small birth density connector wherein the plug has a key secured to an outside wall by a pair of flanges and the receptacle has a corresponding key secured to the inside surface of a wall so that when mated the two keys are mutually opposed. The receptacle and plug keys each have a groove and projection combination formed therein so that they mate. Such a structure requires that the connector be made wider to accommodate the keys than would otherwise be necessary. And, since all of the keys are on the same side of the connector, there is no provision for prevention of inadvertent mating of the contacts along the opposite edge of the connector. Such inadvertent mating can be harmful to the connector contacts as well as to the circuitry on the circuit board and to related equipment.
What is needed is a connector keying arrangement that effectively prevents inadvertent partial mating of the contacts of a plug with the contacts of a receptacle either along an edge of the connector or an end thereof. Additionally, the keying arrangement should not require that the connector be made larger to accommodate the keys or to require additional space on the circuit board.